This invention proposes a solution to the problem of the thermomechanical stresses encountered in the flexible portions, subject to temperature-induced deformation, of the filters and of the multiplexers, of the known type called OMUX (Output Multiplexer), with thermally-compensated technology resonant cavity and high power.
Generally, and hereinafter in the description and in the claims, the expression “thermally-compensated technology” is used to mean any technology that aims to deform a resonant cavity by temperature so as to compensate the volume variation of said resonant cavity, said volume variation being induced by temperature changes, so as to keep the resonance frequency of the cavity at the desired value. This value is generally predefined in ambient temperature conditions in the region of 20° C.
It will be recalled that a microwave resonator is an electromagnetic circuit tuned to let energy at a precise resonance frequency pass. The microwave resonators can be used to produce filters in order to reject the frequencies of a signal located outside the pass band of the filter.
A resonator takes the form of a structure forming a cavity, called resonant cavity, the dimensions of which are defined to obtain the desired resonance frequency.
Thus, any change to the dimensions of the cavity that introduce a change of volume of said cavity will cause a shift in its resonance frequency and, consequently, a change in its electrical properties.
The changes in the dimensions of a resonant cavity may be due to expansions or contractions of the walls of the cavity caused by temperature changes, which become all the more significant if the thermal expansion ratio of the material increases, and/or as the temperature variation increases.
A number of thermo-compensation techniques are known.
These techniques rely more often than not on the combination of parts involved in the structure of the cavity itself and that are made of materials with different thermal expansion ratios, one of the ratios being much lower than the other. The parts are arranged in such a way as to generate temperature-induced displacements relative to one another by exploiting the thermoelastic differential effect. Coupled with a flexible wall, they cause a deformation in the sense of a volume reduction when the temperature increases, or a volume increase when the temperature decreases.
Conventionally, a first material with a very low thermal expansion ratio, such as Invar™, is used. The second material used is normally aluminium, a material that has a higher thermal expansion ratio than Invar and that has, in addition to a low density, a high thermal conductivity, making it particularly well suited to space applications.
Based on this same principle of the use of two materials with different thermal expansion ratios, there are various compensation devices external to the cavity, the role of which is to deform a flexible wall.
Some of these temperature-compensation devices are, for example, described in the Patent Applications EP1187247 and EP1655802.
In order to meet the increasingly strong constraints in arranging satellite payloads, vertical channel architectures, that is to say, for example, architectures that have superposed input and output cavities, have been developed. These architectures are particularly detrimental from the point of view of the thermal control of the channel.
Now, in a hot environment, that is to say at temperatures of the order of 85° C. in the field of space applications, and faced with increasingly high dissipated power levels, that is to say above 100 Watts dissipated in an OMUX filter, the compensated technologies may have usage limitations.
In practice, to meet the needs for compensation, that is to say for deformations beyond 200 microns of displacement at the centre of the cap, the cap must be made sufficiently flexible and deformable to keep the material in its elastic domain.
The flexibility can be obtained in the case of a circular cap by increasing the distance between the rigid circular portion at the centre and the outer rigid circular portion, or even by reducing the thickness of the membrane.
In both cases, this has the effect of making the cap more thermally resistive, and consequently greatly reducing the local thermal gradients, that is to say at the place of the flexible wall itself.
High gradients may be particularly detrimental, for example with the use of aluminium alloys with structural hardening, such as aluminium 6061, the mechanical properties of which can decrease very rapidly as a function of the temperature and the duration of exposure to this same temperature. The temperature, and therefore the thermal resistance, must consequently be limited.
Conversely, to favour the reduction of the thermal gradients in the membrane, the thickness of the flexible portion can be increased, or the distance between the rigid portion at the centre and the outer rigid circular portion can be reduced, but then, the flexibility of the cap reduces, and may consequently become incompatible with the need for deformation to achieve the requisite compensation.
A first solution could involve using more thermally conductive materials, but these are generally incompatible with regard to their mechanical properties, or even with regard to their thermoelastic properties in conjunction with the structure of the aluminium resonant cavity.
To reduce the thermal gradients, the most obvious solution involves increasing the thickness of the walls of the OMUX filters, in order to favour the heat flux conducted towards the thermal control system of the satellite payload.
Now, this solution may become prohibitive for the competitiveness of the product, particularly in space applications because of the resulting significant weight increase.
The present invention resolves these difficulties by proposing a system that is compatible with different compensation solutions, and that makes it possible to reduce the thermal gradient of a flexible cap by a significant factor, and one that affects the overall weight only by a few grams.
The present invention therefore complements the current thermo-compensation technologies for filters and OMUX with resonant cavities. It relates more specifically to the flexible caps of thermally-compensated OMUXs. The idea is to optimize the ratio between the thermal resistance and the deformability of said caps.
Thus, to obtain a lower thermal resistance of the flexible caps, while maintaining deformability, the invention proposes a multiple-membrane flexible wall system. This system may also make it possible to reduce the mechanical stresses for a given deformation, while retaining an equivalent thermal resistance, or even to increase the deformation for equivalent levels of mechanical stresses and thermal resistance, and therefore to maintain equivalent thermal gradients for a given dissipated power.